We are standing up a new data center and users received SMTP errors while testing. We were able to ping the server so DNS lookup and network traffic was working. But we only had a generic SMTP error message. So, I fired up telnet and tested our SMTP connection using these commands:
telnet o smtp.psadmin.io ehlo mail from:email@example.com rcpt to:firstname.lastname@example.org data Test Email .
When we first tested send emails through the SMTP server, we received an “Unable to relay for email@example.com” error. That’s much more descriptive and a useful message to send to the Exchange administrators.